The long term objectives of this project are to understand the origin and functional significance of Ly-1 (CD5+) B cells. These cells show numerous distinctions from the "conventional" set of B cells, including an apparent predisposition to anti-self responses. Indeed, the human counterpart of this population appears involved in human diseases with autoimmune aspects, such as rheumatoid arthritis and Sjogren's syndrome. Furthermore, in both mouse and man it appears that this subset contributed disproportionately to B cell neoplasms. The immediate goals of this project are to define the relationship of Ly-1 B to the bulk of Ly-1 negative (conventional) B cells and to determine the mechanism for enrichment of auto-reactivities in this population. We have developed a robust system for isolating progenitors (Pro B) committed to generating either Ly-1 B or conventional B cells and now we seek to: 1) establish the earliest differentiation stage at which commitment to Ly-1 B occurs by analysis of SCID mice repopulated with cell fractions earlier than Pro B; 2) investigate the importance of microenvironment in commitment to one or the other pathway of B lymphopoiesis by transfer of adult stem cells into fetal recipients; 3) carry out transfer experiments with immunoglobulin transgenic mice to test opposing model for the induction of Ly-1 on B cells; and 4) define genetic differences between fetal and adult Pro B cells by RT-PCR analysis. We have also investigated specificities uniquely enriched in Ly-1 B, particularly the anti- bromelain-treated mouse erythrocyte autoantibodies encoded largely by the VH11 gene family. We will extend our work in this area by: 1) using a VH11-JH4 PCR analysis to define the earliest timepoint at which biased expression of VH11 is detectable in Ly-1 B; 2) sequencing VH11 rearrangements in bone marrow to determine whether they are productive or show any of the restrictions noted in the adult Ly-1 B repertoire; and 3) generating cDNA libraries to determine immunoglobulin V gene family usage in both primary and peripheral populations generated from purified fetal liver and bone marrow Pro B cells. This work may eventually help us to understand the relationship of special features of fetal B cell development with autoreactivity nd B cell neoplasia.